1. Field of the Invention
The present invention relates to a thin-film capacitor to be used for a hybrid microwave integrated circuit and to a method of manufacturing a hybrid microwave integrated circuit using the thin film capacitor.
2. Description of the Prior Art
In the field of telecommunication, it is a tendency to make the signal frequency higher and higher, and especially in the field of satellite communication, the signal frequency will be made 10 GHz or higher. Being accompanied by such a tendency, miniaturization of equipment or devices to be used in such a higher frequency band is needed. In the same way, a low cost integrated circuit having good characteristics in a microwave band is strongly needed.
A hybrid microwave integrated circuit or a monolithic microwave integrated circuit has been used as a microwave integrated circuit for amplifying or modulating an electric signal in the microwave band of 10 GHz to 30 GHz. The hybrid microwave integrated circuit is constituted with transistors, resistors and capacitors which are mounted on a substrate. The constitution and the manufacturing method of a typical microwave hybrid integrated circuit are disclosed in M. Caulton, "Microwave Integrated-Circuit Technology - A Survey", IEEE Journal of Solid State Circuits, Vol. sc-5, No. 6, pp. 292-303 (1970). In this case, thin-film resistors of tantalum nitride and thin-film capacitors of silicon oxide are integrated on a single-crystalline substrate of sapphire etc. In hybrid microwave integrated circuits which are actually used at present, in most cases, thin-film capacitors are not used and chip capacitors are used. The reason that the chip capacitor is used in a hybrid microwave integrated circuit instead of the thin-film capacitor is mainly the restriction required by the substrate. Usually an alumina substrate is used as the substrate, which has low loss in a microwave band, a proper value of dielectric constant, an excellent resisting property against heat and chemicals and is low in cost. The alumina substrate has excellent electric properties and is low in cost, but since it is a polycrystalline sintered body, the occurrence of defects on the surface of it cannot be avoided. Even after mirror finish, there are many large surface defects, and the surface is not smooth or flat enough to form thereon a thin-film capacitor. If a sapphire single-crystalline substrate is used the surface is very smooth and flat, so that a thin film capacitor can be formed, but its cost is as high as ten times that of an alumina substrate. From the economical point of view, the sapphire substrate is not preferable.
As a method of solving the problem, are is a well known method in which the surface of an alumina substrate is covered with a vitreous film of 2 or 3 .mu.m to make the surface flat and smooth, and thin-film resistors and thin-film capacitors are formed on it. The vitreous film is inferior in heat and chemical resistivity to the alumina substrate, which restricts an integrating process. For example, tantalum nitride is often used as a thin-film resistor for its high reliability, and an etchant of hydrofluoric acid series is usually used for the etching of tantalum nitride. The vitreous film is generally affected by chemicals of hydrofluoric acid series, so that if a substrate with a vitreous film is used it becomes difficult to integrate a thin-film resistor using a tantalum nitride. By using a dry etching method in which fluid etchant is not used, etching can be performed without giving damage to the substrate. However, the device for dry etching is expensive and the process lacks in productivity compared with the wet etching.
Because of the reasons as mentioned in the above, an alumina substrate and chip capacitors are generally used in the hybrid microwave integrated circuit. In the case of this constitution, the number of parts is increased, and the joining joints by wire bonding or by an adhesive are increased. This inevitably increases manufacturing processes and causes cost increase, which degrades the degree of integration. The joining points by wire bonding or by adhesives are weak against mechanical vibrations or temperature changes, which can cause degradation of reliability. The connection by wire bonding causes parasitic reactance which degrades high frequency characteristics.
The monolithic microwave integrating circuit is constituted on a substrate of silicon or GaAs etc. on which active elements such as transistors and various kinds of passive parts are integrated. It is suited to mass production in comparison with the hybrid microwave integrating circuit, but it has drawbacks such as large substrate loss, poor heat discharge characteristic, difficulty in adjustment. Therefore, the hybrid microwave integrated circuit and the monolithic microwave circuit are used selectively in proper way depending on the usage.
The hybrid microwave integrated circuit is often used in the case where impedance matching is difficult, low loss is needed and good heat dissipation is needed, as in the case of a solid state power amplifier.
As a thin-film capacitor to be used for an integrated circuit many kinds of materials, manufacturing methods and structures are proposed and actually being used. A typical thin-film capacitor and its manufacturing method and disclosed in U.S. Pat. No. 3,679,942. (Metal-Oxide-Metal, Thin-Film Capacitors and Method of Making Same", issued on June 25, 1972 by Daly). In this method, silane (SiH.sub.4) is chemically reacted with oxygen to make silicon oxide and it is deposited on a single-crystalline crystalline substrate of sapphire, magnesium-aluminate spinel or silicon. It is a method so called a chemical vapor-phase deposition method. The specification of this patent describes that the silicon oxide thus formed is improved much in its dielectric loss characteristic by heat treatment for more than six hours at a temperature of 395-425.degree. C. With this method good quality thin-film capacitors can be manufactured with good yield when a single crystalline substrate which has a flat and smooth surface is used, but when a ceramic substrate such as an alumina substrate is used troubles can occur. It is because of the fact that the withstand voltage and the production yield of the thin film capacitor largely depend on the existence of defects and their sizes on the surface of the substrate.
The dielectric constant of silicon oxide is about 4, and assuming the thickness of silicon oxide be 1 .mu.m, the capacitance of a thin-film capacitor of 1 square mm is 35 pF. The capacitors to be used in a microwave integrated circuit are shown in the following: the capacity of a DC blocking capacitor is 1-3 pF, that of an impedance matching capacitor is 0.5-5 pF, that of a bypass capacitor is 10-100 pF. Therefore the necessary area for a thin-film capacitor is in the range of about 200 .mu.m square to about 2 mm square. The probability that defects are caused in the part where a thin-film capacitor is formed is very high, and the short circuit between electrodes is apt to occur in this part, so that production yield and withstand voltage characteristic are much deteriorated. To increase the capacitance per unit area, when the film thickness is made thinner the area of a capacitor can be made small but withstand voltage characteristic per unit area is deteriorated. When the film thickness is increased withstand voltage characteristic per unit area is improved but the area of a capacitor has to be increased, which disturbs miniaturization and the degree of integration.
When a capacitor is made by a chemical vapor-phase deposition method, there is a limit in the film thickness to be formed. In an ordinary method when the film thickness grows more than about 2 .mu.m fine cracks begin to occur on the film and an electric characteristic is degraded. The thickness limit is practically about 5 .mu.m. Therefore the film thickness of silicon oxide thus formed is 1000 .ANG.-5 .mu.m. On the other hand, there are 2 or 3 pieces of defects per square mm which are large enough in size and depth in comparison with the film thickness of 1-5.mu.m on the surface of an ordinary alumina substrate even after mirror finish. Such large defects as described above cannot be covered with a chemical vapor-phase deposition film, in the edge part especially the film thickness becomes extremely thin and a short circuit between electrodes is apt to occur in this part. There is a so called thick film capacitor which can be used in an integrated circuit other than the thin-film capacitor. This type of capacitor is made by a method wherein a dielectric material is mixed with a binder or a solvent to make it a paste, and a thick film is formed on a substrate with it by a method of screen printing etc and it is baked at a high temperature. A thick film obtained in this way generally has a thickness of several scores of microns and so it is called a thick film capacitor. A thick film capacitor with an excellent withstand voltage characteristic can be easily obtained but the loss in a microwave band is large. Therefore the thick film capacitor is not used in the frequency range of more than 1 GHz.